Plot a tick mark (a vertical line) for each value of I<vname> that is
non-zero and not *UNKNOWN*. The I<fraction> argument specifies the length of
the tick mark as a fraction of the y-axis; the default value is 0.1 (10% of
Plot a tick mark (a vertical line) for each value of I<vname> that is
non-zero and not *UNKNOWN*. The I<fraction> argument specifies the length of
the tick mark as a fraction of the y-axis; the default value is 0.1 (10% of